1. Field of the Invention
The present invention relates generally to a slip control system for controlling engine driving torque so as to maintain vehicular wheel slippage at an optimal level for optimizing vehicular driving performance. More specifically, the invention relates to a slip control system which employs a map or table for deriving slip control characteristics in response to a variation of operation magnitude of an accelerator depending upon vehicular driving conditions.
2. Description of the Background Art
In general, it is essential to maintain good road/tire traction for obtaining good vehicular driving performance. For this, wheel slippage has to be maintained within a predetermined optimum range so as to optimize transmission of driving torque from the tread of a vehicular driving wheel to the road surface. Adjustment of driving torque to be delivered to the road wheel is particularly important when the vehicle passes through a road having a surface with substantially low friction, such as wet road, or icy road. In order to maintain wheel slippage at optimum level, Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-43133 proposed a slip control system, in which fuel supply amount for the engine is controlled according to an accelerator position. The disclosed system also includes an arithmetic circuit for comparing rotation speeds of a driving wheel and a driven wheel and for deriving a wheel slippage based on the difference therebetween. When wheel slippage is greater than a predetermined wheel slippage threshold, fuel supply amount is forcingly reduced irrespective of the accelerator position. In such conventional slip control, the fuel supply amount is increased when wheel traction is recovered and thus wheel slippage is decreased to be lower than the wheel slippage threshold, fuel supply amount is resumed to the normal value corresponding to the accelerator position. This tends to cause increased wheel slippage again and thus cause hunting of slip control. Hunting of slip control causes driving torque fluctuations causing jerking on the vehicular body, thereby degrading riding comfort of the vehicle. Furthermore, since the fuel supply amount is controlled irrespective of the accelerator position during slip control mode operation, drive feeling can be degraded because the engine speed is not adjusted linearly corresponding to the accelerator position.